1. Field of the Invention
The present invention is related to power and control systems for heaters, for example, to apparatus and methods for powering and controlling multiple heaters used for heating pipes and other components in vacuum, process, delivery, transport, and other systems, developed by HPS Division of MKS Instruments, Inc., and Watlow Electric Manufacturing Company, parties to a joint research agreement (35 U.S.C. 103I(2)C)).
2. State of the Prior Art
Many vacuum, process, delivery, transport, and other systems used in industry for conducting or moving various gaseous, liquid, or solid materials from one point to another include pipes of various lengths, sizes, and shapes that have to be heated to maintain the pipes and/or materials in the pipes within certain temperature ranges. Pipe heaters for heating pipes for these and other purposes are well known to persons skilled in the art and have ranged from simple resistive wires and tape wrapped around the pipes to more sophisticated, insulated pipe heaters, such as those described in U.S. Pat. No. 5,714,738 (Hauschultz et al.), which is incorporated herein by reference, as well as many such heater products that are available commercially.
Along with the development of pipe heaters for various pipe heating applications, there was also a need for better pipe heater control systems for regulating heat output from the heaters along lengths of pipe and for monitoring and controlling such heater operations. There are many kinds and configurations of such heater control systems, such as the ones described in U.S. Pat. No. 6,894,254 (Hauschultz), which is also incorporated herein by reference. As good as such heater monitoring and control systems are, however, there are still problems that they have not solved.
For example, in higher temperature installations, the heat produced by the pipe heaters can be conducted to heat controller components that are mounted directly on the pipe heaters, thereby potentially raising the temperatures of such controller components to levels that can damage or destroy them or that can corrupt or degrade data in logic circuits or memories in the controller systems. Some power control systems are hard wired to heater components of the systems making it difficult to quickly replace them. Also, most industrial pipe heaters are equipped with thermal high limit fuses or thermal activated switches that cut the power to pipe heaters if the temperature reaches a maximum temperature threshold, regardless of the cause, for the safety of personnel, to prevent damage to capital equipment, and for safety agency certification. This function has been provided with a variety of thermal limit devices, none of which are entirely satisfactory for this application.
For example, standard, commercially available thermal switches are inaccurate and unreliable due to their wide set point tolerances and contact mechanisms, which can erode or, even worse, self-weld to a closed position that renders them totally inoperative and can allow a thermal runaway of the heater until either the heater element burns out or starts a fire. These problems are exacerbated when the thermal switches are placed in or on the heaters where they need to be for accurate response to the actual temperature of the heater and pipes, because the high heat at the heater is a major cause for such degradation of the thermal switches. Yet, the thermal switches cannot be placed off or away from the heaters, because they would not be able to respond to actual temperatures of the heaters or pipes.
Thermal fuses are more dependable and available commercially, but once they expire, i.e., “blow” or “burn out”, they cannot be reset. Since thermal fuses are typically embedded in the pipe heater structure near the heating element to be sure they are exposed to the heat near its source, they are not accessible without destructive mutilation of the heater components and materials. Therefore, a blown or burned out thermal fuse renders the heater completely useless so it has to be replaced. Also, thermal fuses age over time, and the higher the temperatures to which they are exposed, the faster they age. Such aging often causes thermal fuses to burn out at lower temperatures and eventually to burn out within the normal operating range of the pipe heaters, thus rendering the otherwise good pipe heaters unusable. Also, commercially available thermal fuses are bulky and difficult to install in pipe heaters.
There are sometimes circumstances that cause the temperatures of pipes, thus of the pipe heaters, to exceed such upper temperature limits that have nothing to do with a runaway or uncontrollable heater. For example, it is not uncommon to purge or clean process chambers upstream from the pipe systems by sending high temperature gases or reactive chemicals through them, which can cause the pipe temperature, thus also the pipe heater temperature, to temporarily exceed the upper temperature limit and thereby cause the thermal fuse to expire and open the power circuit to disable the heater. When the thermal fuse expires and cannot be reset or replaced, good heaters are ruined by such routine maintenance and other occurrences unrelated to the pipe heaters themselves.
Also, there is a need for more options and versatility in both connection and control configurations to accommodate a wider variety of piping configurations, applications, and user requirements. Each pipe installation is different and many operators need custom pipe heater and control systems to accommodate their particular requirements, but designing and manufacturing custom pipe heater systems is expensive, time consuming, and often not feasible for most applications. For example, some operators want a control mechanism for each heater in a heated pipe system, whereas other operators prefer to avoid the cost of individual controls on each heater and instead use a strategy wherein a single controller is used to operate an entire zone comprising a number of individual heaters. Such “zoning” or “single point” control heater systems often require complex wiring, which can create confusion and increases the probability of wiring errors, or it can require custom heaters to be designed and built to accommodate slaving and prevent wiring error, which adds costs and complexity to the system.
Another example is that some operators require remote communications with heater controllers and remote heater system control capabilities so that they can view operating status information and modify operating parameters from a remote location, whereas others want to be able to view such operating status information and to modify operating parameters locally at each heater within a system. Still others require only basic, pre-programmed control at each heater. Of course, there are also operators who want any combination or all of these functions for a group of heaters with only single point control.
These and other requirements in industrial and commercial use of pipe heaters creates a need for a more flexible system of pipe heater controls and wiring components that can be configured easily, neatly, and effectively to meet a wider variety of operator requirements.